The present invention relates generally to joining sheet material, and more particularly to a molded stripper for use in the joining apparatus to form leak-proof or non-leak-proof joints.
The ability to permanently join multiple pieces of sheet material through punching or other manipulation and deforming them into an interlocking relationship has found widespread applications. In this regard, such method and apparatuses for permanently joining sheet metal or other sheet material items, with the capability of forming either leak-proof joints or conventional xe2x80x9clanced jointsxe2x80x9d has been the subject of numerous patents. Such apparatuses include die members that are movable laterally, transverse to the longitudinal movement of a punch against an anvil between the dies which are relatively simple but durable in construction, which utilize standard or specialized punches, which are compact and suitable in many different applications, which preserve the corrosive resistance of coated sheet material being joined, and which are suitable for use either in a small press or in a C-frame holder of a large press. In this regard, U.S. Pat. No. 5,581,860 and the related U.S. patents identified therein are exemplary of this technology. The disclosure of all such patent applications and issued U.S. patents are expressly incorporated by reference herein.
In pertinent part to the present invention, U.S. Pat. No. 5,581,860 discloses a punch assembly having a punch body mounting a circular punch. The circular punch includes a threaded portion which receives and threadingly supports a stripper retainer. Disposed within the stripper retainer is a stripper or sheet metal retainer biased to a stripping position by means of a coil spring. The stripper serves to clamp sheet material together before and during the formation of the joint. The stripper preferably has an outside diameter at the area of longitudinal engagement with the sheet metal to be formed that is greater than the inside diameter of the die opening when the dies are at their maximum laterally outward open position. This, coupled with biasing forces urging the stripper longitudinally against the sheet metal surrounding the joint in order to clamp the sheet metal pieces together, greatly reduces the tendency for the sheet material to flow longitudinally outward, away from the preferably fixed anvil inside of the die opening.
The coil spring used to generate the stripper biasing force has a tendency to degrade during repeated cycling. More specifically, the clamping forces generated by the stripper decrease as the stripper is repeatedly cycled, and may ultimately fail in a fatigue mode. Likewise, the metal tip of the stripper which contact the sheet material may scratch or mar the material being stripped. Although the prior art strippers perform satisfactorily, the present application is directed to additional improvements and refinements thereupon.
In accordance with the present invention, as defined in the appended claims, a composite urethane stripper is provided for use in conjunction with various punch assemblies to provide sufficient force to clamp the sheet materials tightly together before and during the forming of the joint. As presently preferred, the composite stripper utilizes two components of urethane having different hardnessxe2x80x94an end portion utilizing a relatively hard durometer urethane and a middle portion utilizing a relatively soft durometer urethane. The configuration of the stripper is such that the clamping force may be accurately and repeatedly controlled.
These features, along with others discussed in more detail below provide a stripper for use in die assemblies of forming apparatuses that are more durable and reliable, more versatile, more stable, more widely applicable, and that typically require less maintenance to operate than previous strippers.
These and other objects, features and advantages of the present invention will become apparent from the subsequent description and the appended claims, taking in conjunction with the accompanying drawings.